Illumination system of a microlithographic projection exposure apparatus

ABSTRACT

An illumination system of a microlithographic projection exposure apparatus includes a spatial light modulator which varies an intensity distribution in a pupil surface. The modulator includes an array of mirrors that reflect impinging projection light into directions that depend on control signals applied to the mirrors. A prism, which directs the projection light towards the spatial light modulator, has a double pass surface on which the projection light impinges twice, namely a first time when leaving the prism and before it is reflected by the mirrors, and a second time when entering the prism and after it has been reflected by the mirrors. A pupil perturbation suppressing mechanism is provided that reduces reflections of projection light when it impinges the first time on the double pass surface, and/or prevents that light portions being a result of such reflections contribute to the intensity distribution in the pupil surface.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of, and claims priority under 35 USC120 to, U.S. application Ser. No. 15/270,201, filed Sep. 20, 2016, nowU.S. Pat. No. 9,933,706, which is a continuation of, and claims priorityunder 35 USC 120 to, U.S. application Ser. No. 14/743,017, filed Jun.18, 2015, now U.S. Pat. No. 9,454,085, which is a continuation of, andclaims priority under 35 USC 120 to, U.S. application Ser. No.13/625,072, filed Sep. 24, 2012, now U.S. Pat. No. 9,091,945, which is acontinuation of, and claims priority under 35 USC 120 to, internationalapplication PCT/EP2010/002780, filed May 6, 2010, the entire contents ofthese applications which are incorporated herein by reference.

FIELD

The disclosure generally relates to an illumination system of amicrolithographic projection exposure apparatus, and in particular to anillumination system which includes an array of reflective elements usedas a spatial light modulator.

BACKGROUND

Microlithography (also referred to as photolithography or simplylithography) is a technology for the fabrication of integrated circuits,liquid crystal displays and other microstructured devices. The processof microlithography, in conjunction with the process of etching, is usedto pattern features in thin film stacks that have been formed on asubstrate, for example a silicon wafer. At each layer of thefabrication, the wafer is first coated with a photoresist which is amaterial that is sensitive to radiation, such as deep ultraviolet (DUV)or extreme ultraviolet (EUV) light. Next, the wafer with the photoresiston top is exposed to projection light in a projection exposureapparatus. The apparatus projects a mask containing a pattern onto thephotoresist so that the latter is only exposed at certain locationswhich are determined by the mask pattern. After the exposure thephotoresist is developed to produce an image corresponding to the maskpattern. Then an etch process transfers the pattern into the thin filmstacks on the wafer. Finally, the photoresist is removed. Repetition ofthis process with different masks results in a multi-layeredmicrostructured component.

A projection exposure apparatus typically includes an illuminationsystem for illuminating the mask, a mask stage for aligning the mask, aprojection objective and a wafer alignment stage for aligning the wafercoated with the photoresist. The illumination system illuminates a fieldon the mask that may have the shape of a rectangular or curved slit, forexample.

Ideally, the illumination system illuminates each point of theilluminated field on the mask with projection light having a welldefined intensity and angular distribution. The term angulardistribution describes how the total light energy of a light bundle,which converges towards a particular point on the mask, is distributedamong the various directions of the rays that constitute the lightbundle.

The angular distribution of the projection light impinging on the maskis usually adapted to the kind of pattern to be projected onto thephotoresist. For example, relatively large sized features may involve adifferent angular distribution than small sized features. The mostcommonly used angular distributions of projection light are referred toas conventional, annular, dipole and quadrupole illumination settings.These terms refer to the intensity distribution in a system pupilsurface of the illumination system. With an annular illuminationsetting, for example, only an annular region is illuminated in thesystem pupil surface. Thus there is only a small range of angles presentin the angular distribution of the projection light, and thus all lightrays impinge obliquely with similar angles onto the mask.

Different ways are known to modify the angular distribution of theprojection light in the mask plane so as to achieve the desiredillumination setting. For achieving maximum flexibility in producingdifferent angular distribution in the mask plane, it has been proposedto use mirror arrays or other spatial light modulators that illuminatethe pupil surface.

In EP 1 262 836 A1 the mirror array is a micro-electromechanical system(MEMS) including more than 1000 microscopic mirrors. Each of the mirrorscan be tilted about two orthogonal tilt axes. Thus radiation incident onsuch a mirror device can be reflected into almost any desired directionof a hemisphere. A condenser lens arranged between the mirror array andthe pupil surface translates the reflection angles produced by themirrors into locations in the pupil surface. This known illuminationsystem makes it possible to illuminate the pupil surface with aplurality of spots, wherein each spot is associated with one particularmirror and is freely movable across the pupil surface by tilting thismirror.

Similar illumination systems are known from US 2006/0087634 A1, U.S.Pat. No. 7,061,582 B2 and WO 2005/026843 A2.

However, using a mirror array in the illumination system can alsoinvolve re-designing the illumination system to some extent. Forexample, the use of a mirror array typically involves an additional beamfolding mechanism such as prisms or plane folding mirrors to keep theoverall dimensions of the illumination system small.

In this context US 2009/0116093 A1 proposes a special prism thatincludes a first surface and a second surface at which impingingprojection light is reflected by total internal reflection. The firstsurface reflects the projection light towards a surface from which theprojection light leaves the prism and falls on the mirror array. Theprojection light reflected from the mirror array enters the prism againvia this surface and impinges on the second surface. From there it isdirected towards a condenser lens arranged between the prism and a pupilsurface of the illumination system. Therefore the prism is similar to aconventional K prism with the exception that light is coupled out of theprism and coupled into the prism the surface through which theprojection light passes twice. In a conventional K prism, the prismangle formed between the first and the second reflecting surfaces isdifferent so that also this surface reflects all the light by totalinternal reflection.

Using a prism instead of mirrors for beam folding purposes can beadvantageous because at present the reflective coatings of mirrors have,for the wavelengths typically used in microlithographic illuminationsystems, a reflectivity which is not substantially above 95%, whereasthe process of total internal reflection results in a reflectivity ofnearly 100%.

However, in the pupil surface of the illumination system disclosed inthe afore-mentioned US 2009/0116093 A1 the light intensity distributionis often not satisfactory. In particular, there are undesired lightcontributions to the intensity distribution in the pupil surface. Theselight contributions can perturb the angular light distribution ofprojection light illuminating the mask.

SUMMARY

The disclosure provides an illumination system including a reflectivespatial light modulator and a beam folding unit such that undesiredlight contributions to the intensity distribution in the pupil surfacecan be prevented or at least significantly reduced.

In one aspect, the disclosure provides an illumination system of amicrolithographic projection exposure apparatus including a lightsource, which is configured to produce projection light, and a pupilsurface. The illumination system further includes a spatial lightmodulator which is configured to vary an intensity distribution in thepupil surface. The spatial light modulator includes an array ofreflective elements which are configured to reflect impinging projectionlight into directions that depend on control signals applied to thereflective elements. A beam folding unit including at least one prismdirects the projection light produced by the light source towards thespatial light modulator. The at least one prism has a double passsurface on which the projection light impinges twice, namely a firsttime when leaving the at least one prism and before it is reflected bythe reflective elements, and a second time when entering the at leastone prism and after it has been reflected by the reflective elements.The illumination system further includes a pupil perturbationsuppressing mechanism which is configured to reduce reflections ofprojection light when it impinges the first time on the double passsurface, and/or to prevent that light portions being a result of suchreflections contribute to the intensity distribution in the pupilsurface.

The disclosure is based on the realization that the light portionsperturbing the desired intensity distribution in the pupil surface arecaused by reflections of projection light at the double pass surface. Iflight leaves the prism by passing through the double pass surface undera non-zero angle of incidence, a small fraction of the light isreflected. Usually such reflections are of little concern inillumination systems because the reflected light will be absorbed bysome components of the illumination system so that it reduces the amountof light which is available for the illumination of the mask, but doesnot perturb the illumination of the mask as such.

However, in the case of the double pass surface used in the at least oneprism of the beam folding unit, light reflected at the double passsurface would usually not impinge on some absorbent component of theillumination system, but would be directed towards the pupil surface.More particularly, the reflected light would emerge from the prism ascollimated light which is focused by a subsequent condenser right intothe center of the pupil surface.

Such an undesired light contribution to the intensity distribution inthe center of the pupil surface (i.e. on the optical axis) can haveparticularly adverse effects in the case of certain non-conventionalillumination settings, for example annular, dipole or quadrupoleillumination settings. With these illumination settings the center ofthe pupil surface shall not be illuminated at all. But also forconventional illumination settings which involve an illumination of thecenter of the pupil surface, the contributions from the light reflectedat the double pass surface may compromise the quality of the pupilillumination as a result of interference effects.

To suppress such perturbations of the pupil illumination, the disclosureproposes to either reduce the reflection of projection light at thedouble pass surface, and/or to prevent that light portions being aresult of such reflections contribute to the intensity distribution inthe pupil surface.

In one embodiment the pupil perturbation suppressing mechanism includesan anti-reflective coating which is applied on the double pass surface.Although such anti-reflective coatings usually absorb a certain amountof light, the positive effect of reducing the reflections predominatesthe small light losses which are associated with the use ofanti-reflective coatings.

In another embodiment the pupil perturbation suppressing mechanismincludes diffractive structures that are applied to the double passsurface. By suitably determining the dimensions of the diffractivestructures, it can be achieved that light reflected at the double passsurface will undergo destructive interference so that reflections arereduced.

According to another embodiment the pupil perturbation suppressingmechanism includes a mechanism ensuring that the angle of incidence ofthe projection light, when it impinges the first time on the double passsurface, equals the Brewster angle. This approach exploits the fact thatlight in a p-polarization state is not reflected at an optical surfaceif it impinges under the Brewster angle. Even if the light impinging onthe double pass surface has no preferred state of polarization, thismeasure will reduce the amount of reflected light by 50%. This reductionincreases the higher the degree of polarization is for light in ap-polarization state when it impinges the first time on the double passsurface. Therefore it is preferred that at least 80%, and morepreferably at least 95%, of the projection light is in a p-polarizationstate when it impinges the first time on the double pass surface.Ideally 100% of the projection light is in a p-polarization state,because then no light at all will be reflected at the double passsurface.

A polarizing unit may be provided in the polarization system thattransforms an initial state of polarization, which the projection lighthas when it impinges on the polarizing unit, into a p-polarizationstate. Such a polarizing unit may include a half-wave plate, aquarter-wave plate and at least two birefringent plates having anon-uniform thickness. With such a polarizing unit any arbitrary spatialdistribution of defined polarization states may be transformed into ap-polarization state.

According to still another embodiment the pupil perturbation suppressingmechanism includes a liquid which fills an interspace formed between thedouble pass surface and the reflective elements of the spatial lightmodulator. Such a liquid significantly reduces the reflections at thedouble pass surface, because it brings the refractive index ratio at thedouble pass surface close to 1. If the refractive index of the liquidequals the refractive index of the at least one prism, no reflections atall occur at the double pass surface.

According to a further embodiment at least one prism includes a firstreflective surface and a second reflective surface. The pupilperturbation suppression mechanism includes a mechanism ensuring thatthe double pass surface forms different angles with the first and secondreflective surfaces so that the light portions being a result ofreflections at the double pass surface do not reach the pupil surface.The oblique orientation of the double pass surface breaks up thesymmetry of the at least one prism which is the basic reason why lightreflected at the double pass surface of the at least one prism isdirected towards the center of the pupil surface.

According to a still further embodiment the pupil perturbationsuppressing mechanism includes a mechanism ensuring that the double passsurface is arranged at such a distance from the spatial light modulatorthat areas on the double pass surface, on which projection lightimpinges the first time, are completely separated from areas on thedouble pass surface, on which the projection light impinges the secondtime. Since the undesired reflections only occur on the area of thedouble pass surface on which the projection light impinges the firsttime, the spatial separation of this area from the area on which theprojection light impinges the second time makes it possible to preventreflected light from reaching portions of the prism through which alsoprojection light reflected by the spatial light modulator propagates.

According to yet another embodiment the pupil perturbation suppressingmechanism includes an obscurator which is configured to be inserted intoa light path of the projection light between the prism and the pupilsurface at a position such that it obstructs the light portions being aresult of reflections at the double pass surface so that they do notreach the pupil surface. This measure, which is only useful forillumination settings for which the center of the pupil surface shall bedark, removes the undesired light portions at the latest possible stage,i.e. immediately before they reach the pupil surface.

The obscurator may include a light intensity sensor that is configuredto detect the intensity of projection light impinging on it. Then theprojection light, or at least a part of it, that would otherwise be lostby absorption on the obscurator is used to provide information on theintensity of the projection light. In this context it should be notedthat illumination system usually contain a light intensity sensor formonitoring the intensity of projection light produced by the lightsource. Such a light intensity sensor has to branch off a small but notnegligible amount of projection light. However, if such a lightintensity sensor is used as an obscurator in the context of thedisclosure, there is no (or at least a reduced) additional light lossincurred by the obscurator. Using a monitor light intensity sensor asobscurator involves the light intensity at the position of the sensor isproportional to the intensity produced by the light source, or at leastthat the dependency between the two intensities being known. Thiscondition is usually fulfilled.

According to another aspect, the disclosure provides an illuminationsystem of a microlithographic projection exposure apparatus including alight source, which is configured to produce projection light, and apupil surface. The illumination system further includes a spatial lightmodulator which is configured to vary an intensity distribution in thepupil surface. The spatial light modulator includes an array ofreflective elements which are configured to reflect impinging projectionlight into directions that depend on control signals applied to thereflective elements. A beam folding unit is provided that includes atleast one prism. The beam folding unit directs the projection lightproduced by the light source towards the spatial light modulator. The atleast one prism has a double pass surface on which the projection lightimpinges twice, namely a first time when leaving the prism and before itis reflected by the reflective elements, and a second time when enteringthe prism and after it has been reflected by the reflective elements. Inaccordance with this second aspect the illumination system furtherincludes a light intensity detector which detects the total intensity ofthe projection light at a position between the light source and a maskto be illuminated. The illumination system further includes a controlunit which controls the spatial light modulator depending on the totalintensity measured by the light intensity detector.

According to this second aspect of the disclosure no measures may betaken that prevent the light portions being a result of reflections atthe double pass surface from reaching the pupil surface. Instead, thesecontributions to the intensity distribution in the pupil surface areaccepted, but taken into account computationally. More particularly, thereflective elements of the spatial light modulator are controlled insuch a way that the real intensity distribution obtained in the pupilsurface equals the desired intensity distribution. Usually this willmean that some of the reflective elements do not direct light towardsthe center of the pupil surface (as it would be in the absence of suchundesired reflections) because these reflections already contribute tothe illumination of this area. In this context it is assumed that theintensity of the light portions being a result of undesired reflectionsat the double pass surface are proportional to the total intensity ofthe light measured by the light intensity detector. It is to be noted,however, that this approach functions well only for conventionalillumination settings or for non-conventional illumination settings forwhich also the center of the pupil surface shall be illuminated.

Instead of assuming a proportionality between the total light intensityand the intensity of the contributions originating from the undesiredreflections at the double pass surface, the intensity of thesecontributions may also directly be measured at the pupil surface byarranging the light intensity detector in the system pupil surface. Thelight intensity detector may be inserted into the beam path duringdisruptions of the projection operation, or a small fraction of thelight may be coupled out of the beam path using a semi-transparentmirror, for example.

BRIEF DESCRIPTION OF THE DRAWINGS

Various features and advantages of the disclosure may be more readilyunderstood with reference to the following detailed description taken inconjunction with the accompanying drawing, in which:

FIG. 1 is a perspective and considerably simplified view of a projectionexposure apparatus in accordance with the disclosure;

FIG. 2 is a meridional section through an illumination system which ispart of the apparatus shown in FIG. 1;

FIG. 3 is an enlarged meridional section through the spatial lightmodulating unit shown in FIG. 2 including an anti-reflective coatingapplied on the double pass surface;

FIG. 4 is an enlarged meridional section through a spatial lightmodulating unit according to a second embodiment including diffractivestructures applied on the double pass surface;

FIG. 5 is an enlarged meridional section through a spatial lightmodulating unit according to a third embodiment in which the projectionlight impinges on the double pass surface under the Brewster angle;

FIG. 6 is an enlarged meridional section through a spatial lightmodulating unit according to a fourth embodiment including a liquidfilling an interspace between the double pass surface and the mirrors ofthe spatial light modulator;

FIG. 7 is an enlarged meridional section through a spatial lightmodulating unit according to a fifth embodiment in which the double passsurface is obliquely arranged;

FIG. 8 is an enlarged meridional section through a spatial lightmodulating unit according to a sixth embodiment in which the distancebetween the double pass surface and the spatial light modulator isincreased;

FIG. 9 is a meridional section through an illumination system accordingto a seventh embodiment in which a light obscurator is used to preventthat reflected light portions reach the center of the pupil surface.

DESCRIPTION OF PREFERRED EMBODIMENTS I. General Construction ofProjection Exposure Apparatus

FIG. 1 is a perspective and highly simplified view of a projectionexposure apparatus 10 including an illumination system 12 which producesa projection light beam. The projection light beam illuminates a field14 on a mask 16 containing minute structures 18. In this embodiment theilluminated field 14 has the shape of a ring segment. However, othershapes of the illuminated field 14, for example rectangles, arecontemplated as well.

A projection objective 20 images the structures 18 within theilluminated field 14 onto a light sensitive layer 22, for example aphotoresist, which is supported by a substrate 24. The substrate 24,which may be formed by a silicon wafer, is arranged on a wafer stage(not shown) such that a top surface of the light sensitive layer 22 isprecisely located in an image plane of the projection objective 20. Themask 16 is positioned via a mask stage (not shown) in an object plane ofthe projection objective 20. Since the latter has a magnification β with|β|<1, a minified image 14′ of the structures 18 within the illuminatedfield 14 is projected onto the light sensitive layer 22.

During the projection the mask 16 and the substrate 24 move along a scandirection which coincides with the Y direction indicated in FIG. 1. Theilluminated field 14 then scans over the mask 16 so that structuredareas larger than the illuminated field 14 can be continuouslyprojected. Such a type of projection exposure apparatus is oftenreferred to as “step-and-scan tool” or simply a “scanner”. The ratiobetween the velocities of the substrate 24 and the mask 16 is equal tothe magnification β of the projection objective 20. If the projectionobjective 20 inverts the image (β<0), the mask 16 and the substrate 24move in opposite directions, as this is indicated in FIG. 1 by arrows A1and A2. However, the disclosure may also be used in stepper tools inwhich the mask 16 and the substrate 24 do not move during projection ofthe mask.

II. General Construction of Illumination System

FIG. 2 is a meridional section through the illumination system 12 shownin FIG. 1. For the sake of clarity, the illustration of FIG. 2 isconsiderably simplified and not to scale. This particularly implies thatdifferent optical units are represented by one or very few opticalelements only. In reality, these units may include significantly morelenses and other optical elements.

The illumination system 12 includes a housing 28 and a light source 30that is, in the embodiment shown, an excimer laser. The light source 30emits projection light having a wavelength of about 193 nm. Other typesof light sources 30 and other wavelengths, for example 248 nm or 157 nm,are also contemplated.

In the embodiment shown, the projection light emitted by the lightsource 30 enters a beam expansion unit 32 in which the light beam isexpanded. The beam expansion unit 32 may include several lenses or maybe a mirror arrangement, for example. The projection light emerges fromthe beam expansion unit 32 as an almost collimated beam 34.

The projection light beam 34 then enters a spatial light modulating unit36 that is used to produce a variable intensity distribution at a pupilsurface 38. Various embodiments of the spatial light modulating unit 36will be described in more detail below with reference to FIGS. 3 to 9.

Between the spatial light modulating unit 36 and the pupil surface 38 acondenser 40 is arranged which transforms the different directions ofthe light rays emerging from the spatial light modulating unit 36 intodifferent locations at the pupil surface 38. In other embodiments thecondenser 40 is dispensed with so that the spatial light modulating unit36 directly illuminates the pupil surface 38 in the far field.

In or in close vicinity to the pupil surface 38 an optical integrator 42is arranged which includes two optical raster elements 44, 46 which mayinclude arrays of cylindrical lenses or fly's eye lenses, for example.The optical integrator 42 produces a plurality of secondary lightsources that each illuminate, via a further condenser 48, anintermediate field plane 50 in which a field stop 52 is arranged. Thefurther condenser 48 assists in superimposing the light bundles, whichhave been emitted by the secondary light sources, in the intermediatefield plane 50. Due to this superposition a very uniform illumination ofthe intermediate field plane 50 is achieved. The field stop 52 mayinclude a plurality of moveable blades and ensures, to the extentdesired, sharp edges of the illuminated field 14 on the mask 16.

A field stop objective 54 provides optical conjugation between theintermediate field plane 50 and a mask plane 56 in which mask 16 isarranged. The field stop 52 is thus sharply imaged by the field stopobjective 54 onto the mask 16.

III. Spatial Light Modulating Unit

In the following various embodiments of the spatial light modulatingunit 36 will be described. As will become apparent, the spatial lightmodulating units 36 differ from one another mainly with respect to apupil perturbation suppressing mechanism incorporated therein.

1. First Embodiment—Anti-Reflective Coating

FIG. 3 is an enlarged meridional section through the spatial lightmodulating unit 36 shown in FIG. 2. The spatial light modulating unit 36includes a spatial light modulator 58 and a prism 60.

The spatial light modulator 58 includes a mirror array 62 which, inturn, includes a plurality of small individual mirrors 64 that can betilted, independently from each other, by two tilt axes that arepreferably aligned perpendicularly to each other. The mirror array 64may be a microelectromechanical system (MEMS), and in particular as adigital micro-mirror device (DMD).

The spatial light modulator 58 further includes a mirror control unit 66which controls the tilting movements of the mirrors 64 and is connectedto an overall system control (not shown) of the illumination system 12.Actuators that are used to set the desired tilt angles of the mirrors 64receive control signals from the mirror control unit 66 such that eachindividual mirror 64 is capable of reflecting an impinging light ray bya reflection angle that is variable in response to the control signal.In the embodiment shown there is a continuous range of tilt angles atwhich the individual mirrors 64 can be oriented. In other embodimentsthe actuators are configured such that only a limited number of discretetilt angles can be set.

Instead of a mirror array 62 an array of other reflective elements maybe used that are configured to reflect impinging projection light intodirections that depend on control signals applied to the reflectiveelements. Such elements may include, for example, electro-optical oracousto-optical cells. In such cells the refractive index may be variedby exposing a suitable material to electric fields or ultrasonic waves,respectively. These effects can be exploited to produce index gratingsthat direct impinging light into various directions.

The prism 60 has generally the shape of a K prism, although it has adifferent function. More specifically, the prism 60 has a pair ofopposite flat surfaces, namely a light entry surface 68 and a light exitsurface 70. The prism 60 further includes two flat reflective surfaces,namely a first reflective surface 72 and a second reflective surface 74.The two reflective surfaces 72, 74 are arranged at a prism angle β withrespect to each other.

Opposite the two reflective surfaces 72, 74 a flat surface 76 extends ina plane which is arranged perpendicular to the light entry surface 68and the light exit surface 70. For reasons which become apparent belowthis surface will be referred to in the following as double pass surface76.

In the embodiment shown the prism 60 consists of a material which has ahigh transmittance for the projection light. For projection light havinga wavelength of 193 nm, calcium fluoride (CaF₂) may be used as opticalmaterial because it has a higher transmittance than fused silica orother glasses. Reducing transmission losses is not only important withregard to the throughput of the projection exposure apparatus 10, butalso avoids problems caused by heat which is created inside the prism 60by the absorption of projection light.

The prism 60 may be produced from a single piece of material, but mayalso be assembled from individual smaller prisms. For example, the upperand the lower half of the prism 60 shown in FIG. 3 may be formed by twoindividual prisms that each includes one of the reflective surfaces 72or 74. Furthermore, the prism 60 may have additional edges and surfaces.For example, those portions through which no projection light passes maybe completely dispensed with to reduce the costs of the opticalmaterial.

In the following the function of the spatial light modulating unit 36will be explained:

The projection light beam 34, which is at least substantiallycollimated, enters the prism 60 substantially perpendicularly throughits light entry surface 68 and is then completely reflected at its firstreflective surface 72 by total internal reflection. Total internalreflection occurs because the prism angle β formed between the tworeflective surfaces 72, 74 is selected such that the angle of incidenceof the projection light on the first reflective surface is equal to orgreater than the critical angle α_(c). For calcium fluoride (n≈1.50 forλ=193 nm) the critical angle α_(c) is about 42°.

After being reflected at the first reflective surface 72 the projectionlight beam 34 impinges on the double pass surface 76. At this surfacethe angle of incidence is smaller than the critical angle α_(c) so thatthe greatest portion of the projection light 34 leaves the prism 60 andimpinges on the tiltable mirrors 64 of the mirror array 62. The mirrors64 reflect the impinging projection light into directions that depend onthe control signals supplied by the mirror control unit 66 to themirrors 64. Generally the tilt angles of the mirrors 64 will not exceeda few degrees so that the largest portion of projection light reflectedby the mirrors 64 is able to enter the prism 60 through the double passsurface 76. Since this is the second time that the projection lightpasses through the surface 76, the latter is referred to here as doublepass surface.

The projection light which now propagates within the prism 60 towardsthe second reflective surface 74 will now have propagation directionswhich depend on the tilt angles of the mirrors 64 from which the lighthas been reflected. However, the directions are still within a rangesuch that the angles of incidence on the second reflective surface 74are equal to or greater than the critical angle α_(c). Thus projectionlight which has entered the prism 60 through the double pass surface 76is completely reflected at the second reflective surface 74 and directedtowards the light exit surface 70. From this surface 70 the projectionlight 34 leaves the prism 60 and the spatial light modulating unit 36 invarious directions. As mentioned above, the condenser 40 will thentranslate the various directions into different locations in the pupilsurface 38 of the illumination system 12.

In the foregoing description of the function of the spatial lightmodulating unit 36 it has been mentioned that the greatest part of theprojection light passes through the double pass surface 76 when itimpinges the first time on it. This implies, however, that a smallportion of the projection light would be reflected at the double passsurface 76. In FIG. 3 a light ray indicated with a broken line 78represents such a reflected light portion. Due to the symmetry of theprism 60, the light reflected at the double pass surface 76 wouldimpinge on the second reflective surface 74 with the angle of incidenceunder which the projection light impinges on the first reflectivesurface 72. Consequently, the reflected light portion 78 would emergefrom the prism 60 parallel to the direction of the incoming projectionlight beam 34. As it is shown in FIG. 2, the condenser 40 would thenfocus the parallel reflected light portion into the center of the pupilsurface 38. There the light portions being a result of reflections atthe double pass surface 76 would produce a light spot and thus perturbthe intensity distribution in the pupil surface 38.

The light portions being a result of reflections at the double passsurface 76 and contributing to the intensity distribution in the pupilsurface 38 have an adverse effect on the angular light distribution ofprojection light illuminating the mask 16. This is particularly true forillumination systems in which no light shall impinge perpendicularly onthe mask 16, which implies that the center of the pupil surface 38 hasto be completely dark. But also for illumination settings in which thecenter of the pupil surface 38 shall be illuminated, such contributionsfrom reflected light may have an adverse effect due to interferences. Inthe following these adverse effects are referred to as pupilperturbations.

In the embodiment of the spatial light modulating unit 36 shown in FIG.3 the reflections of projection light when it impinges the first time onthe double pass surface 76 are reduced by an anti-reflective coating 80which is applied at least on those portions of the double pass surface76 on which the projection light 34 impinges the first time. Theanti-reflective coating 80 therefore suppresses at least to some extentpupil perturbations that may otherwise be caused by projection light 78that is reflected when impinges the first time on the double passsurface 76.

2. Second Embodiment—Diffractive Structures

The spatial light modulating unit 36 shown in FIG. 4 differs from thespatial light modulating unit 36 shown in FIG. 3 only in that theanti-reflective coating 80 is replaced by diffractive structures 82 thatare applied to the double pass surface 76. The dimensions of thediffractive structures 82 are determined in such a way that lightreflected at the diffractive structures 82 interferes destructively.This substantially reduces reflections of projection light 34 when itimpinges the first time on the double pass surface 76.

3. Third Embodiment—Brewster Angle

In the embodiment of a spatial light modulating unit 36 shown in FIG. 5the prism angle β is increased to such an extent that the angle ofincidence α_(i) of the projection light 34 equals the Brewster angleα_(p) when it impinges the first time on the double pass surface 76. Ifcalcium fluoride is used as material for the prism 60 and the wavelengthλ of the projection light is 193 nm, the refractive index n₁ of theprism is 1.5015. Assuming for the refractive index n₀ of the surroundingmedium n₀=1, the Brewster angle α_(p) given by arctan(n₀/n₁) is thenabout 33.66°. If p-polarized light impinges under the Brewster angleα_(p) on the double pass surface 76, reflections are completelysuppressed. If impinging projection light 34 is unpolarized or circulardepolarized, it can be decomposed into one half of s-polarized and onehalf of p-polarized light, so that even then the reflections are reducedby 50%. Generally, the higher the degree of p-polarization is, thesmaller the reflections at the double pass surface 76 will be.

In the embodiment shown the spatial light modulating unit 36 includes apolarizing unit 84 which is arranged between the beam expansion unit 32and the light entry surface 68 of the prism 60. The polarizing unit 84transforms an initial state of polarization, which the projection light34 has when it impinges on the polarizing unit 84, into a p-polarizationstate. To this end the polarizing unit 84 includes a half-wave plate 86,a quarter-wave plate 88 and two birefringent plates 90, 92 having anon-uniform thickness distribution. With such an arrangement of platesit is possible to transform any arbitrary spatial distribution of linearor elliptical states of polarization into a p-polarization state.

If the optical material, from which the prism 60 is made, isbirefringent, this may also be taken into account by the polarizing unit84. For example, if the prism 60 is made of calcium fluoride which isintrinsically birefringent, the orientation of the crystal lattice hasto be known such that for each light ray the retardance caused by theintrinsic birefringence can be computed and taken into account. In thiscase it may also be envisaged to separate the prism 60 into two or moresmaller prisms whose crystal lattices are arranged in such a manner thatthe retardances produced in each piece compensate each other at least tosome extent.

Behind the light exit surface 70 of the prism 60 a further polarizingunit 84′ may be arranged that transforms the state of polarization,which the projection light 34 has after it has been reflected by themirrors 64 and propagated again through the prism 60, into any desiredstate of polarization. The further polarizing unit 84′ may also includea half-wave plate 86′, a quarter-wave plate 88′ and two birefringentplates 90′, 92′ having a non-uniform thickness.

4. Fourth Embodiment—Liquid

FIG. 6 shows a spatial light modulating unit 36 which includes a liquid94 filling an interspace formed between the double pass surface 76 andthe mirrors 64. To this end the spatial light modulating unit 36includes a casing 96 which surrounds this interspace and ensures thatthe liquid 94 remains in place. The liquid 94 may be circulated withinthe interior of the casing 96 by a pump (not shown). Furthermore, atemperature controller may be provided that ensures a constanttemperature of the liquid 94.

If the liquid 94 is a high index liquid, the refractive index ratio ofthe liquid 94 and the material of the prism 60 may approach 1 so that nolight is reflected at the double pass surface 76. Even if water having arefractive index of about 1.4 is used as the liquid 94 and calciumfluoride having a refractive index of about 1.50 is used as material forthe prism 60, the refractive index ratio is very close to 1 so thatreflections of projection light impinging the first time on the doublepass surface 76 are significantly reduced.

5. Fifth Embodiment—Oblique Double Pass Surface

FIG. 7 shows a fourth embodiment of a spatial light modulating unit 36in which no measures are taken to reduce reflections at the double passsurface 76. However, the light portions resulting from such reflectionsare prevented from reaching the pupil surface 38.

To this end the spatial light modulating unit 36 of this embodimentincludes a prism 60 having a double pass surface 76 which formsdifferent angles with respect to the first and the second reflectivesurfaces 72, 74. More particularly, the angle formed between the doublepass surface 76 and the first reflective surface 72 is smaller than theangle formed between the double pass surface 76 and the secondreflective surface 74. By suitably selecting these angles it can beachieved that projection light, which is reflected at the double passsurface 76 when it first impinges the first time on it, does not reachthe pupil surface 38.

In FIG. 6 broken lines 78 a, 78 b indicate such light portions being aresult of reflection at the double pass surface 76. The reflected ray 78a impinges on the second reflective surface 74 with such a large angleof incidence that it can be blocked out of the light path after it hasleft the prism 60. The other ray 78 b impinges with such a large angleof incidence on the light exit surface 70 that it is reflected by totalinternal reflection. Thus none of the rays 78 a, 78 b contributes to theintensity distribution in the pupil surface 38.

6. Sixth Embodiment—Spatial Separation

Also in the embodiment of a spatial light modulating unit 36 shown inFIG. 8 no measures are taken to reduce reflections at the double passsurface 76, but to prevent light portions resulting from suchreflections from reaching the pupil surface 38.

In this embodiment the mirror array 62 of the spatial light modulator 58is arranged at a larger distance away from the double pass surface 76.This larger distance has the effect that areas on the double passsurface 76, on which the projection light 34 impinges the first time,are completely separated from areas on the double pass surface 76, onwhich the projection light impinges the second time. Then light portions78 reflected at the double pass surface 76 cannot impinge on the secondreflective surface 74, but are reflected at the first reflective surface72 and do not reach the pupil surface 38.

7. Seventh Embodiment—Pupil Obscurator

FIG. 9 is a schematic meridional section similar to FIG. 2 through anillumination system 12 according to another embodiment. Also in thisembodiment no measures are taken that reduce reflections at the doublepass surface 76.

In FIG. 9 light portions being a result of such reflections areindicated by broken lines 78. In order to prevent that these lightportions contribute to the intensity distribution in the pupil surface38, a light obscurator 98 is arranged immediately in front of the pupilsurface 38. The light obscurator 98 is configured in this embodiment asa small circular plate which absorbs all impinging projection light. Thelight obscurator 98 is held in a central position in the pupil surface38 by three thin wires 100 angularly separated by 120°. The wires 98 andthe obscurator 98 can be removed from the light path using anappropriate manipulator.

If the illumination system 12 shall produce an illumination setting forwhich the center of the pupil surface 38 has to be completely dark, theobscurator 98 is inserted into the light path so that it obstructs thelight portions 78 being a result of reflections at the double passsurface 76.

If the illumination system 12 shall produce an illumination system forwhich also the center of the pupil surface 38 has to be illuminated, theobscurator 98 is removed from the light path. Then the light portions 78being a result of reflections at the double pass surface 76 are allowedto contribute to the intensity distribution in the pupil surface. Thiscontribution is computationally taken into account, and the mirrorcontrol unit 66 ensures that less mirrors 64 direct projection lightinto the center of the pupil surface 38 than would be if there were nolight portions 78.

The obscurator 98 may include, and in particular may be completelyformed by, a light intensity sensor that is configured to detect theintensity of projection light impinging on it. The output signal of thelight intensity sensor may then be used to monitor the intensity of theprojection light produced by the light source 30. This involves thelight intensity at the position of the obscurator 98 having a knowndependency from the intensity produced by the light source 30.

It is to be understood that the measures taken in the embodimentsdescribed above to suppress pupil perturbation resulting fromreflections at the double pass surface can also be combined in variousways. Generally, if measures are taken to reduce reflections at thedouble pass surface 76, there will be still some—albeit verysmall—reflected light portions 78 that may ultimately reach the pupilsurface 38. To completely eliminate the pupil perturbations caused bythese reflected light portions, the light obscurator 98 may be insertedinto the beam path for illumination settings that are completely darkpupil center.

The above description of the preferred embodiments has been given by wayof example. From the disclosure given, those skilled in the art will notonly understand the disclosure and its attendant advantages, but willalso find apparent various changes and modifications to the structuresand methods disclosed. The applicant seeks, therefore, to cover all suchchanges and modifications as fall within the spirit and scope of thedisclosure, as defined by the appended claims, and equivalents thereof.

1. (canceled)
 2. An illumination system for directing light along alight path, the illumination system comprising: a light modulatorpositioned in the light path, the light modulator being configured tovary an intensity distribution of incident light propagating along thelight path; an optical element positioned in the light path, the opticalelement having a surface on which the light impinges a first time and asecond time, the first time being when the light leaves the opticalelement and before the light is reflected from the light modulator, thesecond time being when the light enters the optical element after thelight has been reflected by the light modulator; and a perturbationsuppressing mechanism configured to: a) reduce reflections of the lightwhen the light impinges on the surface of the optical element the firsttime; and/or b) prevent reflections of the light when the light impingeson the surface of the optical element the first time from contributingto the intensity distribution, wherein the illumination system is amicrolithographic illumination system.
 3. The illumination system ofclaim 2, wherein the perturbation suppressing mechanism comprises ananti-reflective coating supported by the surface.
 4. The illuminationsystem of claim 3, wherein the perturbation suppressing mechanismcomprises diffractive structures supported by the surface.
 5. Theillumination system of claim 4, wherein the perturbation suppressingmechanism comprises a mechanism configured to ensure that an angle ofincidence of the light when the light impinges the first time on thesurface is the Brewster angle.
 6. The illumination system of claim 3,wherein the perturbation suppressing mechanism comprises a mechanismconfigured to ensure that an angle of incidence of the light when thelight impinges the first time on the surface is the Brewster angle. 7.The illumination system of claim 2, wherein the perturbation suppressingmechanism comprises diffractive structures supported by the surface. 8.The illumination system of claim 7, wherein the perturbation suppressingmechanism comprises a mechanism configured to ensure that an angle ofincidence of the light when the light impinges the first time on thesurface is the Brewster angle.
 9. The illumination system of claim 2,wherein the perturbation suppressing mechanism comprises a mechanismconfigured to ensure that an angle of incidence of the light when thelight impinges the first time on the surface is the Brewster angle. 10.The illumination system of claim 9, wherein at least 80% of the light isin a p-polarization state when it impinges the first time on thesurface.
 11. The illumination system of claim 10, further comprising apolarizing unit positioned in the light path, the polarizing unit beingconfigured to transform the light when it impinges on the polarizingunit from an initial state to a p-polarization state.
 12. Theillumination system of claim 9, further comprising a polarizing unitconfigured positioned in the light path, the polarizing unit beingconfigured to transform the light when it impinges on the polarizingunit from an initial state to a p-polarization state.
 13. Theillumination system of claim 12, wherein the polarizing unit comprises:a half-wave plate; a quarter-wave plate; and at least two birefringentplates having a non-uniform thickness.
 14. The illumination system ofclaim 2, wherein the perturbation suppression mechanism comprises aliquid between the surface and the light modulator.
 15. The illuminationsystem of claim 14, wherein, along the light path, the surface and thelight modulator are separated only by a space and the liquid fills theentire space between the surface and the light modulator.
 16. Theillumination system of claim 2, wherein the surface of the opticalelement is a twice-traversed surface and the optical element comprises afirst reflective surface in the light path and a second reflectivesurface in the light path in addition to the twice-traversed surface,and the perturbation suppression mechanism comprises a mechanismconfigured to ensure that the twice-traversed surface forms differentangles with the first and second reflective surfaces so that the lightportions resulting from reflections at the twice-traversed surface donot contribute to an irradiance distribution at a pupil surface of theillumination system.
 17. The illumination system of claim 2, wherein theperturbation suppression mechanism comprises a mechanism configured toensure that the surface is a distance from the light modulatorsufficient that areas on the surface on which the light impinges thefirst time are completely separated from areas on the surface on whichthe light impinges the second time.
 18. The illumination system of claim2, wherein the perturbation suppression mechanism comprises anobscurator insertable into the light path behind the optical element sothat the obscurator obstructs portions of the light resulting fromreflections at the surface so that the reflections do not contribute toan irradiance distribution at a pupil surface of the illuminationsystem.
 19. The illumination system of claim 18, wherein the obscuratorcomprises a light intensity sensor configured to detect an intensity oflight impinging on the obscurator.
 20. The illumination system of claim2, further comprising a light source.
 21. The illumination system ofclaim 2, wherein directions of the light reflected by the lightmodulator depend on control signals applied to the light modulator. 22.The illumination system of claim 2, wherein the light modulatorcomprises an array of reflective elements configured to reflectimpinging light into variable directions.
 23. The illumination system ofclaim 2, wherein the illumination system has a pupil surface, and thelight modulator is configured to vary the irradiance distribution in thepupil surface.
 24. The illumination system of claim 2, wherein theoptical element is a prism.
 25. An apparatus, comprising: anillumination system for directing light along a light path, comprising:a light modulator positioned in the light path, the light modulatorbeing configured to vary an intensity distribution of incident lightpropagating along the light path; an optical element positioned in thelight path, the optical element having a surface on which the lightimpinges a first time and a second time, the first time being when thelight leaves the optical element and before the light is reflected fromthe light modulator, the second time being when the light enters theoptical element after the light has been reflected by the lightmodulator; and a perturbation suppressing mechanism configured to: a)reduce reflections of the light when the light impinges on the surfaceof the optical element the first time; and/or b) prevent reflections ofthe light when the light impinges on the surface of the optical elementthe first time from contributing to the intensity distribution; and aprojection objective, wherein the apparatus is a microlithographicprojection exposure apparatus.
 26. A method of using a microlithographicprojection exposure apparatus comprising an illumination system and aprojection objective, the method comprising: using the illuminationsystem to direct light along a light path to illuminate a mask havingfeatures with the light; and using the projection objective to projectat least some of the features of the mask onto a photoresist, whereinthe illumination system comprises: a light modulator positioned in thelight path, the light modulator being configured to vary an intensitydistribution of incident light propagating along the light path; anoptical element positioned in the light path, the optical element havinga surface on which the light impinges a first time and a second time,the first time being when the light leaves the optical element andbefore the light is reflected from the light modulator, the second timebeing when the light enters the optical element after the light has beenreflected by the light modulator; and a perturbation suppressingmechanism configured to: a) reduce reflections of the light when thelight impinges on the surface of the optical element the first time;and/or b) prevent reflections of the light when the light impinges onthe surface of the optical element the first time from contributing tothe intensity distribution.